Infrared or thermal imaging systems typically use a plurality of thermal sensors to detect infrared radiation and produce an image capable of being visualized by the human eye. Thermal imaging systems detect thermal radiance differences between objects in a scene and display these differences in thermal radiance as a visual image of the scene. Certain infrared radiation characteristics must be dealt with to obtain a satisfactory image. One such consideration is the background radiance from the scene which is frequently a large component of the total infrared radiation emitted by the scene. Radiance differences among the objects are often small as compared to the large value of background radiance.
The basic components of a thermal imaging system typically include optics for collecting and focusing infrared radiation from a scene, an infrared detector having a plurality of thermal sensors for converting the radiance to an electrical signal, and electronics for amplifying and processing the electrical signal into a visual display or for storage in an appropriate medium. Such thermal imaging systems use a variety of infrared detectors which are sometimes classified into two main categories as cooled and uncooled. Uncooled detectors include thermal sensors which generate a change in voltage due to a change in temperature resulting from incident infrared radiation striking the thermal sensor. Cooled detectors include thermal sensors which generate a change in voltage due to a photoelectron interaction within the material used to form the thermal sensor. This latter effect is sometimes called the internal photoelectric effect.
An infrared detector typically generates a signal in response to radiation emitted by a viewed scene. Either "staring" or scanning techniques may be used to produce the signal, but in either case, the signal has the same characteristics. The total signal detected by the infrared detector is a biased signal, which includes a direct current (DC) component or signal offset and an alternating current (AC) component or signal modulation. The signal offset represents the background radiation. The signal modulation represents radiance differences emitted by objects within the scene. Yet, because of the large background radiance, the amplitude of the signal modulation is small in comparison to the amplitude of the signal offset, making amplification and detection of the objects within the scene difficult.
Various efforts have been made to isolate the signal modulation from the biased signal. One approach is to establish a reference signal that is close in magnitude to the signal offset, and then to subtract the reference signal from the overall biased signal to obtain an unbiased signal that preferably contains only the signal modulation. One implementation of this approach is to include an optical device that artificially creates a referenced image. An example of one such thermal imaging system uses an image chopper to produce a de-focused image, which represents the background radiance. However, these mechanical image choppers are expensive and add significantly to the complexity of the infrared system.